At present, a variety of user terminal devices are widespread among general users. For example, there is even a data processing terminal which executes a variety of user supports as PDA (Personal Digital Assistance) and communicates its processed data using a PHS (Personal Handy-phone System: a registered trademark) function over the air. Here, a first conventional example of such data processing terminal will be described below with reference to FIG. 1.
First, data processing terminal 10 comprises a hollow housing (not shown), and a parent board 11 disposed inside the housing (not shown). Parent board 11 comprises a large rectangular circuit board which is mounted with a variety of data processing circuits (not shown) comprised of integrated circuits.
Data processing terminal 10 has a user interface (not shown) such as a touch panel formed on the surface of the housing, so that a data processing circuit on parent board 11 executes a variety of data processing in response to data entered through the user interface.
A card slot (not shown) is also formed on one end of the housing, and a separate card-shaped radio communication unit 12 is removably plugged into the card slot. Radio communication unit 12 comprises a radio communication circuit (not shown) contained therein, and a rod-like antenna 13 protruding therefrom. Radio communication unit 12 has a PHS function to communicate with the outside in a frequency band near 1.9 (GHz) over the air.
In data processing terminal 10, as card-shaped radio communication unit 12 is plugged into the card slot of the housing, the radio communication circuit in radio communication unit 12 is connected to the data processing circuit on parent board 11 to establish a wired communication available condition.
One of a pair of connectors 14 is mounted on parent board 11, while the other of the pair of connectors 14 is mounted on child board 15, so that child board 15 is removably mounted on parent board 11 through connectors 14. Child board 15 comprises a small rectangular circuit board which is mounded with a memory circuit (not shown) such as RAM (Random Access Memory), a flash memory, or the like.
Parent board 11 is substantially entirely formed with a first ground plane made of a metal, and child board 15 is likewise substantially entirely formed with a second ground plane made of a metal (not shown), such that these ground planes determine a potential reference of data processing terminal 10.
Connectors 14 have a multiplicity of signal terminals and a multiplicity of ground terminals arranged in parallel with one another, wherein one ground terminal is inserted, for example, per three signal terminals.
Thus, as child board 15 is mounted on parent board 11 through connectors 14, the signal terminals of connectors 14 connect the data processing circuit on parent board 11 to the memory circuit of child board 5, while the ground terminals of connectors 14 connect the first ground plane of parent board 11 to the second ground plane of child board 15.
In the configuration as described above, data processing terminal 10 is responsive, for example, to data entered through a user interface or the like to cause the data processing circuit to execute a variety of data processing and deliver processed data from the user interface, thereby serving as PDA to provide user supports.
Also, since data processing terminal 10 makes radio communications with the outside through radio communication unit 12 as required, data processing terminal 10 can transmit data processed by the data processing circuit over the air, and processes processed data received through radio communications in the data processing circuit. Further, since data processed by the data processing circuit is stored in the memory circuit of child board 15, the data processing circuit can execute a large capacity of data processing.
While data processing terminal 10 herein illustrated has child board 15 mounted on parent board 11 through a pair of connectors 14 which are removable in the vertical direction, there is also a product, as data processing terminal 20 illustrated in FIG. 2, wherein child board 15 is mounted to and removed from connector 21 in the horizontal direction.
Also, as data processing terminal 30 illustrated in FIG. 3, there is a product which comprises parent board 11 and child board 15 formed with ground terminals 31, 32 electrically connected to a first ground plane and a second ground plane, respectively, formed on the front surfaces or back surfaces thereof, with ground terminals 31, 32 being electrically connected through tubular metal columns 34 or screws 35 of auxiliary connecting means 33. Such metal columns 34 and screws 35 are generally intended to mechanically hold child board 15, so that they are disposed near a pair of corners at diagonal positions of rectangular child board 15.
However, in the aforementioned data processing terminal 10, 20, first ground plane 17 of parent board 11 and second ground plane 18 of child board 15 connected through connectors 14, 21 are positioned in parallel with each other, as illustrated in FIG. 4, where data processing circuit 19 composed of a multiplicity of electronic parts such as LSIs (Large Scale Integration), and signal wires is mounted on first ground plane 17 of parent board 11.
Since data processing circuit 19 transmits repetitive signals and non-repetitive signals at particular frequencies among circuits when it processes data, an electromagnetic field is generated near such circuits, associated with frequency components and harmonic components of the transmitted signals. This electromagnetic field not only causes a high frequency current to flow into first ground plane 17 of parent board 11 but also induces a high frequency current into second ground plane 18 of child board 15 disposed in the neighborhood.
The inventors found that the ground structure comprised of first ground plane 17 of parent board 11, second ground plane 18 of child board 15, and the ground terminals of connectors 14 as illustrated in FIG. 4 resembled an antenna element of a quarter wavelength resonant antenna referred to as an “inverted L-shaped antenna” or an “inverted F-shaped antenna” (reference: “Small Antennas” K. Fujimoto, A. Henderson and J. R. James, Research Studies Press, Chapter 2.4).
In this event, second ground plane 18 of child board 15 has an edge close to connector 14 that corresponds to a short-circuited end of the antenna element, and an edge opposite to connector 14 that corresponds to an open end of the antenna element. On this assumption, a strong electromagnetic field is generated around second ground plane 18 due to frequency components of a current induced into second ground plane 18 by data processing circuit 19, which cause quarter wavelength resonance in second ground plane 18, thereby affecting radio communication unit 12.
For example, child board 15 having a memory circuit mounted thereon has edges extending over approximately 25 to 75 (mm), so that the quarter wavelength resonance occurs at frequency in a range of approximately 1 to 3 (GHz). On the other hand, since conventional data processing circuit 19 has a basic frequency around several MHz, its harmonics are also on the order of 100 (MHz). As such, the aforementioned ground structure is conventionally free from the quarter wavelength resonance because harmonic components of data processing circuit 19 are largely lower than the frequency of the quarter wavelength resonance in the ground structure.
Recently, however, the trend of faster processing operations has increased the basic frequency of data processing circuit 19 to several hundred MHz, and its harmonics are also as high as several GHz. For this reason, the aforementioned ground structure suffers from the quarter wavelength resonance, and an electromagnetic field of high intensity generated by the ground structure strongly couples to radio communication unit 12 positioned in the neighborhood to impede its radio communications.
Particularly, since frequency bands available for communications are approximately 1.9 (GHz) in PHS; approximately 800 (MHz), approximately 1.5 (GHz), and approximately 2.0 (GHz) in portable telephone; and approximately 2.4 (GHz) in radio LAN (Local Area Network) and Bluetooth, such radio communications will be impeded by the quarter wavelength resonance of the ground structure if it occurs at frequency in a range of approximately 1 to 3 (GHz) as mentioned above.
While it has been predicted from before that the electromagnetic field generated by data processing circuit 19 affects radio communication unit 12, and countermeasures have been taken therefor, nobody has been able to predict that the operation of data processing circuit 19 on parent board 11 causes child board 15 and the like to act as a resonant antenna, and an electromagnetic field generated thereby affects radio communication unit 12.
Describing the phenomenon in a more specific manner, when radio communication unit 12 makes a radio communication at a predetermined frequency, second ground plane 18 generates a strong electromagnetic field at the communication frequency if distance y from connector 14, 21 of child board 15 to the distal edge thereof is approximate to the quarter wavelength of communication waves, as illustrated in FIGS. 1, 2, 4.
The inventors have also confirmed that when connector 14, 21 is positioned along one edge of child board 15 as illustrated in FIG. 6, second ground plane 18 generates a strong electromagnetic field at a communication frequency even if extent a of a plurality of continuous edges of second ground plane 18 from one end to the other end of connector 14, 21 is approximate to the half wavelength of communication waves.
The cause is different from the cause by which the electromagnetic field is generated at the communication frequency due to the aforementioned distance y approximate to the quarter wavelength, and it is predicted that this is because the intensity of the electromagnetic field is not consistent in the longitudinal direction of connector 14, 21 since the length of child substrate 15 in this direction is neither “0” nor infinity,.
Therefore, in the structure in which connector 14, 21 is positioned along one edge of rectangular child board 15 as illustrated in FIGS. 1, 2, 4, second ground plane 18 generates a strong electromagnetic field at a communication frequency even if distance y from connector 14, 21 to the distal edge of child board 15 is approximate to the quarter wavelength of communication waves, or even if extent a of a plurality of continuous edges of second ground plane 18 from one end to the other end of connector 14, 21 is approximate to the half wavelength of communication waves.
On the other hand, in the aforementioned data processing terminal 30, connector 14 is positioned along one edge of child board 15, and auxiliary connecting means are positioned near a pair of corners at diagonal positions, as illustrated in FIG. 7. Thus, second ground plane 18 generates a strong electromagnetic field at a communication frequency particularly when the longest distance a2 is approximate to the half wavelength of communication waves, out of distances a1, a2 from connector 14 to auxiliary connecting means 33 on a plurality of continuous edges of second ground plane 18.
It is predicted that this is because both connector 14 and auxiliary connecting means 33 act as short-circuited ends, so that second ground plane 18 resonates near the half wavelength of the communication waves. Likewise, in this event, the electromagnetic field generated by the ground structure at a communication wavelength due to the resonance impedes radio communications made by radio communication unit 12 positioned in the neighborhood.
Further, in the aforementioned data processing terminal 10, 20, 30, removably connected radio communication unit 12 is impeded in its radio communications, wherein such impediment to radio communications will occur when radio communication unit 12 is positioned near the ground structure.
Thus, communication failures are likewise experienced even by a radio communication circuit which is connected to a data processing terminal as described above through a connection cable or the like, and communication failures are likewise experienced even by a separate radio communication circuit which is not connected to a data processing terminal as described above but is used in the neighborhood (not shown).
To solve the challenges as mentioned above, child board 15 and second ground plane 18 should be formed to be sufficiently small or large with respect to the communication wavelength. However, since the dimensions of child board 15 are restricted to the size of a memory circuit mounted thereon and have been reduced to a possible extent even at present, child board 15 cannot be thoughtlessly reduced more in size. Also, as a matter of course, as child board 15 is increased in size, data processing terminal 10, 20, 30 are also increased in size, thereby impairing the portability and the like.
It is also possible to place child board 15 within a metal case (not shown) to isolate radio communication unit 12 from a strong electromagnetic field generated by child board 15. However, even with the structure using the metal case for shielding overall child board 15 in this way, the metal case results in an increased size of data processing terminal 10, 20, 30.